A spark plug used in a combustion device such as an internal combustion engine includes, for example, a center electrode which extends in the axial direction, an insulator which is formed on an outer periphery of the center electrode, a cylindrical main fitting which is assembled to an outer side of the insulator, and an earthed electrode which has a proximal end portion thereof jointed to a distal end portion of the main fitting. By applying a high voltage to the center electrode, a spark discharge is generated in a gap formed between the center electrode and the earthed electrode so that a fuel gas is ignited as a result.
Further, to enhance the ignitability, there has been proposed a high-frequency plasma spark plug (hereinafter also simply referred to as “spark plug”) which ignites a fuel gas by generating high-frequency plasma with the supply of high frequency power in place of a high voltage to the gap. In transmitting high frequency power to the spark plug, a coaxial cable provided with an inner conductor and a cylindrical outer conductor which covers an outer periphery of the inner conductor is used. The coaxial cable is used for preventing the reflection of electricity and the radiation of electromagnetic wave noises to the outside and for more reliably transmitting high frequency power to the spark plug.
In connecting the spark plug and the coaxial cable to each other, an end portion of the inner conductor is connected to a terminal electrode, and an end portion of the outer conductor is connected to the main fitting which is earthed by being in contact with a combustion device (see Patent Document 1, for example).